1. Field of the Invention
The present invention generally relates to an apparatus and method useful for inducing blocking in plastic films under known conditions. More specifically, the present invention is directed to an apparatus and method for applying a known force to a plurality of plastic film samples maintained at a predetermined temperature for a predetermined time to study induced blocking in plastic film samples under known laboratory conditions.
2. Description of the Background
Blocking is essentially a sticking together of the surfaces of thermoplastic films or coatings. Accordingly, blocking is an undesirable characteristic. The tendency of thermoplastic films and coatings to suffer blocking increases with elevated temperatures and with increasing contact pressure between adjacent films or coatings.
It is important to be able the blocking tendency of thermoplastic films and coatings under various conditions. Accordingly, methods and procedures have been developed to study blocking in the laboratory. ASTM Standard D-3354 defines blocking as essentially a sticking together of the surfaces of adjacent thermoplastic films or coatings under the exclusion of air. Various ASTM Standards set forth test methods to measure the blocking force, i.e., the force required to separate blocked film surfaces.
The preparation of blocked film samples for testing, however, has traditionally been performed mainly using various non-standardized methods. In a commonly used method weights are merely placed upon film samples in an oven to produce the desired contact stresses in an environment of elevated temperatures. Because these methods are non-standardized, the researcher is often unable to reproduce prior results. Further, the researcher is often unable to correlate his results with those of other workers who have used other testing methods. Reproducibility and correlation of results are poor due to the inability to precisely duplicate placement of the weight on the samples to assure uniform contact stress. In addition to poor reproducibility and correlation, these methods suffer from a variety of other shortcomings. For example, the attainable stress levels are limited by the weights which may be safely handled in a hot environment, by the weight which the oven can support and by the size of the oven.
Accordingly, there has been a long felt but unfulfilled need in the testing industry for apparatus and methods for obtaining high and reproducible levels of compressive force to block samples in a hot oven while requiring only moderate weights and moderate oven space.